1. Field of the Invention
The present invention relates generally to a media handling device in an image forming machine and, more particularly, to a media sheet feeding method for overcoming at least one media handling failure mode.
2. Description of the Related Art
A high capacity media handling device of an image forming machine typically includes a pick mechanism that feeds media a sheet at a time from pick positions to a downstream media process, such as printing, copying and the like, and an elevator that lifts a stack of media sheets so as to place the top of the stack at a selected pick position relative to the pick mechanism. The pick mechanism includes a pick arm pivotally mounted at one end and having a pick roll mounted at its free end that contacts a top sheet of a media stack. The pick roll includes a friction surface, oftentimes in the form of one or more tires. The pick positions may be at any of a plurality of levels that intersect an inclined surface on a wear or restraint dam of an input tray between the upper and lower ends of the dam. The pick mechanism is able to feed media a sheet at a time most reliably when the top of the stack is at any one of the pick positions. When the media sheets are picked one at a time from the stack to supply the media process, the level of the top of the stack decreases and potentially could go below the lowest level of the pick positions. The main function of the elevator is to return the top of the stack toward the upper limit of the pick positions before it goes below the lower limit thereof which may be the same as the lower end of the inclined surface of the dam.
It is generally the case that the timing for lifting the stack by the elevator should be such that the stack is not lifted when the pick mechanism is feeding sheets of media from the stack. It is understood that doing both at the same time could introduce adverse forces on the stack since the pick mechanism is pressing downward on the stack as the elevator is lifting the stack upward. As a result, heretofore the pick mechanism has been the only prime mover that acts on the top media sheet when it is being first moved from the stack and separated from the next sheet below it. The pick mechanism also includes a gear train supported by the pick arm and operable to transmit both a rotational force and a downward or normal force to the pick roll. The amount of the downward or normal force applied through the pick roll impacts the amount of frictional force created between the pick roll and the top most sheet.
In many media handling devices, the pick mechanism assumes a near horizontal orientation relative to the top of the media stack when the top of the stack is at the upper limit of the pick positions. In the near horizontal orientation, the normal force exerted by the pick mechanism on the stack by rotation of the pick roll may be too low to create the frictional force necessary to overcome both the friction between the top most sheet, the next sheet directly beneath it and the force necessary to buckle the top sheet up the dam. This is termed a ‘fails to feed’ failure mode. Also, in many media handling devices, the pick mechanism assumes a maximum downward sloped orientation relative to the top of the media stack when the top of the stack is at the lower limit of the pick positions. In the maximum downward sloped orientation, the normal force exerted by the pick mechanism on the stack by rotation of the pick roll may be too high, creating so much friction that it may cause what is termed a ‘sheet feed stall’ failure mode to occur.
Thus, there is a need for an innovation that will overcome these failure modes due to creation of frictional forces that are too high when the pick mechanism is at the lower limit or too low when the pick mechanism is at the upper limit of the pick position.